Bacterial motility: How do pili pull? Dale Kaiser  Current Biology  Volume 10, Issue 21, Pages R777-R780 (November 2000) DOI: 10.1016/S0960-9822(00)00764-8

Fig. 1 Cartoon interpretation of type IV pilus retraction, as observed in the experiment of Merz et al. [3], and proposed by G. Oster. The pilin monomer is embedded in the inner membrane bilayer with its hydrophilic head in the periplasm. A pre-pilin peptidase, PilD, cleaves the pilin signal sequence. With the help of other assembly proteins, the pilus is extended. After extension is completed, and possibly following a signal from the pilus tip, retraction commences, driven by PilT. The PilT motor is drawn as a hexameric ATPase, a member of the AAA family of motor proteins [10]. PilT is extracted in the membrane fraction [23], and is shown extending into the periplasmic space between the inner and outer membrane. There it is shown embedded in the rigid peptidoglycan layer that envelops the cell and that provides its mechanical support. Because of its hexameric geometry and the location and structure of its nucleotide binding site, PilT may be homologous to the β subunit of F1 ATPase [24,25]. This possibility is reinforced by the magnitude of the retraction force measured by Merz, et al. [3], which is comparable to the force generated by F1[26–28]. The inset illustrates the possible axial power stroke of PilT, derived from the rotary power stroke of the F1 β subunit by a modification of the top half of the protein. Current Biology 2000 10, R777-R780DOI: (10.1016/S0960-9822(00)00764-8)

Fig. 2 Sun et al[4] interpret the behavior of M. xanthus cells that have attached to a polystyrene surface in terms of pilus retraction. Cells are shown attaching by the tips of their pili (tethering). The cells descend to the surface by retracting their pili into end A. Finally the cell lies down on the surface, extending pili from the other end, B. These pili make a new attachment to the surface, then retract so that the cell glides toward the site of their attachment. Current Biology 2000 10, R777-R780DOI: (10.1016/S0960-9822(00)00764-8)